The present invention relates to an image correcting method and an image correcting apparatus for improving the quality of a dot printed image in an image forming apparatus such as a printer and, more particularly, to a dot image correcting method, and apparatus, which stores correction data corresponding to a correction dot pattern in a window, detects while scanning a dot image whether or not the dot pattern in the window coincides with the correction dot pattern, and if the coincidence is detected, corrects the dot image in accordance with the correction data.
In a digital printer such as a laser beam printer, or in a display such as a liquid crystal display, CRT or the like, an image is formed with dots. Usually, the positions of the dots and the dot diameters are controlled so as to remain approximately constant. However, when an oblique line is printed in a dot image, a step called a jaggy appears at the edge portion, and a processing called a smoothing process is performed by detecting a jaggy edge portion in a dot image and correcting the edge portion so as to change smoothly,
FIG. 1 is a view showing the construction of a dot image correcting apparatus for performing the above-described correction. In FIG. 1, reference number 11 denotes an image memory for storing dot image data, and 12 denotes a dot image correcting apparatus. The dot image correcting apparatus 12 comprises, as shown in the Figure, a line buffer 21 that temporarily stores dot image data for a few lines which are stored in the image memory 11, a window register 23 for storing the window portion denoted by reference numeral 22, a coincidence detection circuit 24 that detects whether or not the dot pattern in the window register 23 coincides with a preset correction dot pattern, and a correction data table 25 that stores correction data corresponding to the correction dot pattern and outputs the correction data in accordance with the coincidence detection result from the coincidence detection circuit 24.
The line buffer 21 has the memory capacity for storing image data for a few lines, sequentially stores image data read from the image memory 11, and successively shifts the range of lines to be stored. The window register 23 temporarily stores data for each dot from the line buffer 21 in the range where comparison of the dot pattern is performed, and outputs them to the coincidence detection circuit 24. When the comparison in the coincidence detection circuit 24 has been done, the window register 23 shifts the position of the window to the right by one dot. This operation is repeated from the left end to the right end of the line buffer 21, and when completed, the line buffer 21 erases the image data for the top line and stores the image data for the next line anew. Reference number 23A denotes the dot at the center of the window 23, and this dot is referred to as an observation dot. In other words, the position of the observation dot 23A is successively scanned on the image to detect coincidence with the correction dot pattern. Correction data indicate how to correct the observation dot 23A.
FIGS. 2A and 2B are views showing the correcting method for correcting a jaggy. As shown in FIG. 2A, a jaggy occurs at the dot denoted by reference number 27. In this case, the dot 27 is shifted to the left by half pitch, the dot above the dot 27 is shifted to the left by an amount less than half pitch, and the dot beneath the dot 27 is shifted to the right by an amount less than half pitch. Shifting the position of a dot is performed, for example, in the case of a laser beam printer, by shifting the modulating signal of the laser,
There is a case where the coincidence of a pattern is not detected, and pattern recognition is performed by processing the dot pattern in the window register 23 with an operation circuit 26 to obtain correction data. In such a construction, however, there is a problem that, as the pattern is stored in the operation circuit 26 to detect the coincidence, size of the operation circuit becomes large and requires longer time for operation.
FIG. 4 is a view showing an example of the construction of the window register 23 and the coincidence detection circuit 24 and the correction data table 25 of FIG. 1. The coincidence detection circuit is well known and, therefore, a detailed explanation thereof is omitted, Basically, a signal line in which signal indicating the value of each dot is output is taken out from the window register 23, and is arranged in parallel in the coincidence detection circuit 24 so as to form a first wiring array. A second wiring array with number of lines corresponding to the number of correction dot patterns is formed so as to intersect the first wiring array. In each signal line of the second wiring array, a gate is provided at the intersection corresponding to each correction data pattern, When a dot pattern output from the window register 23 coincides with correction dot pattern of a second wiring, only the output from the wiring is modified and the correction data register 251 storing the corresponding correction data is activated to output the correction data via a selector 252. When there is no coincident correction dot pattern, no correction data register 251 is activated, and therefore no correction data is output. In practice, another signal line is provided for each signal line of the first wiring array, and an inverted signal is applied to this signal line. One of this set of signal lines is connected to the gate of the transistor connected to each signal line of the second wiring array. When the transistor is connected to a high potential, each signal line of the second wiring array is connected to ground via a resistor.
Japanese Unexamined Patent Publication (Kokai) No. 07-205483 discloses an image processing method and apparatus wherein a smoothing process is performed efficiently at high speed by the primary detection of an edge corner and by computing the number of continuous dots extending vertically and horizontally from the primary detection point as starting point to perform correction processing. However, this patent publication discloses only a smoothing process and does not disclose correction of any portion other than jagged portions, nor disclose correction of line width as described later.
Conventionally, only a smoothing process has been performed. However, with recent advances in the performance of an image forming apparatus such as a printer, an improvement in image quality by performing correction processes other than the smoothing process is being increasingly required, In particular, more precise control of the size and position of a dot, than has been possible before, is required for such a repeating pattern as a single thin line, an outlined thin line, a repetition of thin line, and so forth. In the case of an isolated thin line, for example, processing for increasing line width is performed because a printing process may adversely affect the visibility of the line, Thus, in the case of a black line of 1 dot, correction is performed so as to increase the dot diameter, and in the case of white line of 1 dot, correction is performed so as to decrease the dot diameter on both sides of the line. This correction is performed by varying the width of the modulation signal in the case of a laser printer, and by varying the amount of ink in the case of an ink jet printer. In such a case, an isolated thin line is stored as a correction dot pattern and, by comparison with a dot pattern in the window, it is detected whether or not the dot pattern is an isolated thin line.
Japanese Unexamined Patent Publication (Kokai) No. 11-216910 discloses an image forming apparatus which corrects a dot diameter in accordance with a dot pattern. Also in this case, coincidence with the correction dot pattern needs to be detected, and the same coincidence detection circuit as shown in FIG. 4 is used for this purpose.
In order to determine which of the black pattern or the white pattern is to be emphasized at the edge portion of an image, it is necessary to detect whether the pattern is a black-in-white pattern or a outlined white-in-black pattern, and to perform a suitable correction processing accordingly. In this case, a respective correction dot pattern is required so that the number of correction patterns becomes very large.
FIGS. 5A to 5L are views showing examples of necessary correction dot pattern, and show only those correction dot patterns associated with a vertical line. Thus, similar correction dot patterns are necessary for a horizontal line and for an oblique line. If these correction dot patterns are added, the number of correction dot patterns is increased to about ten times as large as when only detection of jaggy portion is performed. In order to be able to accommodate various line widths, it is necessary that the size of the window register is increased and number of dots is also increased. For example, if only jaggy portion needs to be detected, the window register may be 3xc3x973 or 5xc3x975. But, if correction of line width as described above is also to be performed, a window register of size 7xc3x977 or greater must be used, leading to a correction apparatus with a very large-size circuit, and giving rise therefore to the problem of increased cost.
It is an object of the present invention to realize an image correcting method and apparatus that is capable of performing, in addition to a smoothing process correction processing of line width correction in accordance with the condition of the lines, using a small-size circuit and at low cost.
To attain the above-described object, an image correcting method and apparatus of the present invention performs the coincidence detection using a plurality of windows of different shapes, and obtains correction data by synthesizing the result of the coincidence detection.
Thus, the image correcting method of the present invention is a dot image correcting method in which correction data corresponding to correction dot patterns in a window are stored and a determination is made as to whether or not a dot pattern in a window of a dot image coincides with a correction dot pattern and, if the coincidence is detected, correction is performed in accordance with the corresponding correction data, characterized in that there are plural windows and correction data are stored corresponding to a combination of correction dot patterns in various windows, and that coincidence of dot patterns in the plural windows with the combination of the correction dot patterns in various windows is determined, and that, when coincidence is detected, correction is performed in accordance with the corresponding correction data.
The image correcting apparatus of the present invention is a dot image correcting apparatus comprising a correction data storage circuit that stores correction data corresponding to a correction dot pattern in a window, a pattern coincidence detection circuit that detects whether or not a dot pattern in a window of a dot image coincides with a correction dot pattern, and a correction signal generating circuit that performs correction in accordance with the corresponding correction data when coincidence is detected, characterized in that there are plural windows, and that the correction data storage circuit stores correction data corresponding to a combination of correction dot patterns in various windows, and that the pattern coincidence detection circuit comprises a plurality of pattern coincidence detection circuits for detecting whether or not dot patterns in the plural windows coincide with a combination of correction dot patterns.
In accordance with the image correcting method and apparatus of the present invention, plural windows of different shapes are used to share the function of recognizing a dot pattern so that various patterns can be efficiently distinguished using a small-size circuit. For example, by selecting windows such that one of the windows is a conventional small square window and other windows are vertically, horizontally and obliquely elongated windows, respectively, the pattern can be distinguished using the small square window and the line width can be distinguished using the elongated windows, and these results can be synthesized to distinguish the dot pattern. In this way, each of the windows can be made small and, even after combining them all, the overall size of the circuit can be made small, The elongated windows may be combined into one window spreading radially from an observation dot.
Plural windows are set respectively so as to include a common observation dot, and the correction data are data for correcting this observation dot.